Derivative And Financial Instruments (Changes In Fair Value) (Details) $ in Thousands	1 Months Ended	3 Months Ended		12 Months Ended
	Aug. 31, 2017 USD ($)	Mar. 31, 2018 USD ($) item	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Derivative Instruments Gain Loss [Line Items]
Early termination of hedges	$ 3,600
Commodity Contract
Derivative Instruments Gain Loss [Line Items]
Beginning fair value of commodity derivatives		$ 1,231	$ 6,436	$ 6,436
Net gains (losses) on derivatives		(1,937)	6,055
Ending fair value of commodity derivatives		$ (477)		1,231
Number of counterparties | item		4
Oil reserves | Commodity Contract
Derivative Instruments Gain Loss [Line Items]
Net gains (losses) on derivatives		$ (1,939)	5,495	3,284
Net settlements paid (received) on derivative contracts				(6,422)
Net settlements paid (received) on derivative contracts		229
Natural gas | Commodity Contract
Derivative Instruments Gain Loss [Line Items]
Net gains (losses) on derivatives		$ 2	$ 560	663
Net settlements paid (received) on derivative contracts				$ (2,730)